I am sure this is a non-issue. More a wording thing.
If you give a Chaos Lord a Sigil (for a 4+ inv) and then a Mark of Tzeentch for a +1 inv then my thinking is that he gets a 3+ inv.
BUT the wording in mark of Tzeentch is .... if the model does not have an invulnerable save he gets a 6+.
Well, the Lord doesn't come out the box with an inv, so which do i apply first? The mark and then the sigil and end up with a 4+ (wasted points on MOTz). Or apply the sigil an then the mark for a 3+.
The rule for the mark is easy to apply for Termies, but when you are bolting on two invulnerable saves at the same time it gets a little wishy washy.
Sorry if this has been FAQ'd already.
Rich A.